Body filler on bare metal.

[Lou]

Many years ago I was told by several bodymen never to put body filler over bare metal.always prime or seal first.

I learned the hard way in 1961 about Bondo over bare metal when I repaired a small hole behind the rear wheels on my 56 Ford Victoria.

In the 1980s I had a part time job writing estimates for Maaco, people would do their own body work putting filler over bare metal and have us paint the car. Six months later they be back with a car all rust saying we screwed up. (We always put on the sales contract that we didn't do the body work and that the rust would come back.)

Being stuck at home I've had a chance to watch most of the custom builders do great metal work but then put body filler over their work, where the restoration shops
always seal or prime first.

What is it that I'm missing, are there now fillers that don't wick water?

[RICH MUISE]

I have heard that also, but I have always taken areas down to bare metal with rough discs, did whatever bodywork required, and did a thorough cleaning BEFORE applying filler. Primers never went on until bodywork and filling if needed was complete. Yep, if you apply filler (I don't use the Bondo brand) over a rusty area, the rust is going to come back......sorta......it never actually left. That "small hole" you repaired on your Victoria......unless you cut the rusted area out and replaced it with welded-in new sheet metal, you never removed the rust. What you saw from the outside was probably small compared to where the rust actually started on the inside.
Also, a lot of what your hearing may be due to the area you live in. I can leave metal bare here for weeks with no issues. In Connecticut, it'll probably rust overnight, thus the reasoning for getting a primer on asap.
They make fillers to be applied over bare metal, and fillers to be applied over the primer. Some fillers and glazings do both ways, which is what I use. I have applied very light coats of glazing over primers. I'm talking about the "newer" glazings that are like a thin version of body filler, not the don't-ever-use red crap in a small tube.
One more thing I'll add..........materials have come a long way since you and I were youngins. We just didn't have the choices we do now. Do a lot of research and talk to the guys who do it for a living. I was fortunate enough to have a mentor who was a top-level builder with an interest in 57 Fords. As always, prep is the key. Fillers, imho, should never be applied over smooth surfaces.

Just a novice's 2 cents.

[Ford Blue blood]

I have to agree with Rich on this.  A 67 Ranchero I built in 1997 still looks as good as it did the day he took it home ( minus the road rash and a couple of self induced boo boos ).  After patch panels were welded in and worked, the "surface leveling material" was then applied directly to 40 grit sanded bare metal.  Still hasn't fallen off.  The quality of materials varies wildly.  My personal experience is use the best you can get, get it from a paint supply shop and follow his/her advice with respect to surface prep and finish prep.  Purchase on line, crap shoot IMHO.  Never buy from the Zone, Pep Boys, Wally World or NAPA for that matter.  Their stuff just isn't what you would want under that shiny paint.  There is no way to know how long it has set in a storage area or on the shelf.  The stuff a supplier has is fresh.

[gasman826]

  The stuff a supplier has is fresh.

It does get old...especially the hardener.